1. Field of the Invention
This invention relates to an apparatus for controlling machines for making bags or sacks which have been severed by welding from continuous tubular or semitubular films of synthetic thermoplastics. The apparatus comprises at least one pair of pinch rollers for feeding the continuous films, which pinch rollers are driven via a transmission by a motor. The apparatus also comprises a pair of welding jaws for providing the continuous film with seam welds, which may be transverse seam welds or severed transverse seam welds, a second motor for actuating said pair of welding jaws via a drive mechanism, and optional further processing means, such as punching means, perforating cutters and the like, which are provided with drives.
2. Description of the Prior Art
The development of cyclically operating welding machines for making sacks or bags of synthetic thermoplastics has been restricted by the nature of the conventional drive means. Said drive means provided with mechanical control means fully complied with the requirements existing about 20 to 25 years ago because the plastic films to be processed then were in a phase of vigorous development so that objectives of processing technology were in the foreground.
But at the present time the development of the films has reached a rather final stage and further development of the processing machines has also continued so that boundaries can now be recognized. One boundary is imposed by the fact that the presently conventional machines comprise intercoupled drive means so that, e.g., the welding time and the feeding time of the film are interdependent.
Crank drives or crank-and-rocker linkages are normally used to drive the pinch rollers and the welding jaws.
Where a simple crank drive is used, the feeding time is approximately as long as the standstill time, which is determined by the drive which is coupled during the return stroke. In that one case the welding time need not be taken into account in the cycle because it amounts only to a fractional part of the return stroke time.
The production rate can be increased by the use of crank-and-rocker linkages. But also with such mechanisms, in which the duration of the return stroke is shorter than the duration of the forward stroke, the welding time amounts only to a fractional part of the standstill time so that there is also a loss of time, which reduces the production rate. Even where crank-and-rocker linkages are employed, the feeding time will depend on the standstill time and vice versa. Such dependence is due to the ratio which is determined by the design of the transmission. That unsatisfactory dependence will become apparent, e.g., if the machine is designed for optimum feed and standstill times for the making of bags of a given size. If in such a machine the feed length is doubled so that the feeding time will approximately be doubled too, the standstill time will also be doubled although the time required for welding will not be prolonged.
That dependence which is imposed by the coupled mechanical drives can be avoided in that separate drives are provided for the feed movement and for the welding jaws and said drives are separately controlled.
An apparatus of the kind described might be provided with separate motors, consisting, e.g., of stepping motors, for driving the pair of pinch rollers and the welding jaws, respectively, and said motors might be controlled by control means in such a manner that when the pair of pinch rollers have stopped, the last stepping pulse delivered to the stepping motor for driving the welding jaws, that motor will be operated by corresponding stepping pulses for the time which is required for making the seam welds and that time will not be dependent on the feed length and on the time required for feeding the film. When the stepping motor for driving the welding jaws has stopped, the next following stepping pulse can be delivered to the stepping motor for driving the pinch rollers so that the standstill times will not depend on the feeding times.
It has been disclosed in German Patent Specification 17 79 408 that the welding time can be changed in that welding jaws are driven by a crank drive and the means for driving said crank drive is stopped for a predetermined time when the crank drive is at its operating dead center.